The concept that histamine plays a fundamental rôle in the chemical control of the circulation is suggested by a series of observations by Dale and Lewis and their associates.1It is, however, remarkable that the evidence concerning histamine as a vasodilator substance in the physiologic regulation of the circulation is still largely based on indirect evidence and analogical observations. Direct proof for this function of histamine in the animal body is so far not available, and while the value of parallel and "teleologic evidence" in establishing a physiologic concept is considerable, it is recognized that concepts based on indirect evidence may be open to fallacy and therefore cannot be considered as proved. Burn and Dale and their associates2observed a fall in the arterial blood pressure in anesthetized carnivorous animals. Lewis and his associates3presented evidence that the human skin, when subjected to various sorts of stimuli,